Add potatoes to pot with water: Place potatoes in a medium pot or large saucepan. Cover with water by 1-inch and season water with salt. Bring to a boil over medium-high heat.
Cook potatoes until tender: Let gently boil until potatoes are fully tender when pierced with a fork, about 10 - 15 minutes. Drain in a colander and rinse under hot water for about 5 seconds to remove excess starches (which can make potatoes gluey).
Rice Potatoes: Drain potatoes then pass through a food mill or potato ricer into same pot*.
Stir with liquid ingredients: Melt 5 Tbsp butter and add to potatoes along with half and half and sour cream. Stir just to combine while seasoning with salt to taste and adding an extra 1 -2 Tbsp half and half to thin if needed. Cover and set aside if filling isn't yet complete.
Saute veggies: Melt remaining 1 Tbsp butter in a 12-inch (and deep) cast iron skillet. Add onion, carrots, celery and saute until nearly soft, about 6 minutes. Add garlic and saute 1 minute longer.
Cook beef: Add ground beef in chunks and cook and break up beef until no longer pink, about 6 minutes. If you find there's lots of excess fat then dab some up using balled up paper towels held by tongs (leave 1 - 2 Tbsp rendered fat).
Add thickeners: Add flour and tomato paste and saute 1 minute.
Pour in liquids, let reduce: Pour in red wine, beef broth, Worcestershire, thyme and rosemary. Bring to a simmer stirring frequently. Reduce heat to low and let simmer until reduced by about 2/3 (and alcohol scent has cooked off), about 8 - 10 minutes.
Add remaining greens: Stir in peas and parsley and heat through, about 1 minute. Season filling with salt and pepper to taste.
Assemble shepard's pie: Spread beef filling into an even layer. Dollop spoonfuls of mashed potatoes evenly over top then spread into an even layer**.
Decorate: Run a fork along top to create a pattern of wavy rows.
Bake: Move oven rack about 5-inches below broiler and preheat broiler to high. Transfer to oven and let broil until golden brown on top about 4 - 6 minutes (keep a close on it as every oven varies).
Rest: Let rest a few minutes before serving. Garnish with parsley if desired.